Modifies a subclass of Container to automatically add and remove the border-width setting on GtkContainer.
This allows the subclass to ignore the border width in its size request and allocate methods. The intent is for a subclass to invoke this in its class_init function.
handle_border_width is necessary because it would break API too badly to make this behavior the default. So subclasses must “opt in” to the parent class handling border_width for them.
the class struct of a Container subclass